1. Field of the Invention
This invention relates to fluorescent lamps and is directed more particularly to an amalgam assembly including an improved exhaust tubulation, and to a fluorescent lamp including the improved exhaust tubulation.
2. Description of the Prior Art
The light output of fluorescent lamps is critically dependent upon mercury vapor pressure (vapor density) within the lamp envelope. The mercury vapor pressure, in turn, is controlled by the temperature of excess liquid mercury which condenses in the coldest part of the lamp envelope, the so-called “cold spot”. Fluorescent lamps typically include at least one tubulation that has an opening into the interior of the lamp envelope and which, in construction of the lamp, is used as an exhaust tubulation. At completion of manufacture, the exhaust tubulation is hermetically tipped off and the tipped end typically becomes the lamp “cold spot”.
The amalgam is commonly located in the exhaust tubulation cold spot. Such amalgams reduce the mercury vapor pressure relative to that of pure mercury at any given temperature and thereby permit optimum light output at elevated temperatures. Such amalgams also provide a broadened peak in the light output versus temperature curve, so that near optimum light output is obtained over an extended range of ambient temperatures.
When lamps are operated at temperatures lower or higher than the optimum ambient temperature, light output decreases by as much as 30% or more relative to peak value. This is a common occurrence when lamps are operated in enclosed or semi-enclosed fixtures. In addition to reduced light output, the color of the light varies as a result of the varying contribution of blue spectral emission from the mercury vapor in the discharge.
The problem of mercury vapor pressure control under varying temperature conditions is solved, at least in part, through the use of various alloys capable of absorbing mercury from its gaseous phase. Alloys of low temperature melting metals are often placed within fluorescent lamps to amalgamate with excess mercury, and to regulate the mercury vapor pressure within the lamp. Alloys known to be particularly useful in forming amalgams with mercury include a lead-bismuth-tin alloy, a bismuth-indium alloy, a bismuth and tin alloy, and a zinc, indium and tin alloy. Other useful amalgams are formed with pure indium, pure lead, and pure zinc.
The lamp typically is provided with an excess amount of mercury amalgam, that is, more amalgam than is needed to supply the mercury vaporized when the lamp reaches a stabilized operating condition. As the lamp ages, some of the excess amalgam is required to replace the mercury chemically bound elsewhere in the lamp during the life of the lamp.
When an amalgam fluorescent lamp is turned off, the amalgam cools and the mercury vapor within the lamp is gradually absorbed into the amalgam. When the lamp is turned on, the lumen output is significantly reduced until the amalgam is warmed up to a point at which the amalgam emits sufficient mercury vapor to permit efficient lamp operation.
In some types of lamps, particularly electrodeless fluorescent lamps, it is important that the amalgam be prevented from settling within the arc environment in the lamp envelope where the amalgam can cause deleterious changes in the lumen output and the lumen-temperature performance of the lamp, causing a sudden rise in mercury vapor pressure and an increase in lamp voltage, resulting in the occurrence of black spots on the glass envelope. If the lamp voltage exceeds the maximum sustaining voltage of the ballast provided in the lamp, the lamp extinguishes. There is thus required a means for retaining liquid amalgam in the tubulation, but permitting mercury vapor to exit the tubulation and flow into the lamp envelope.
Typically, the exhaust tubulation is pinched inwardly during manufacture of the lamp. At least one glass ball is then placed in the tubulation through an open end of the tube and comes to rest on the pinched portion of the tubulation. An amalgam body is then inserted into the tubulation and comes to rest adjacent the uppermost glass ball. Near the end of the manufacturing process, the lamp envelope is evacuated through the exhaust tubulation. The evacuation of the lamp envelope is sometimes blocked or hampered by the presence of the glass ball at the pinched portion of the tubulation. In time, the glass ball is forced from the tubulation pinched portion sufficiently to allow gas to pass and the lamp to be evacuated. The aforementioned open end of the tube is then sealed, with the amalgam disposed between the newly sealed tubulation end and the glass ball or balls adjacent the tubulation pinched portion.
While the glass balls in due course give way under the pressure of exiting gas and permit evacuation of the lamp envelope and tubulation, it is deemed desirable to provide a tubulation and glass ball arrangement which permits ready evacuation through the tubulation, while retaining the glass ball or balls.
The glass balls serve the function of keeping the solid amalgam spaced from the tubulation pinched portion and, after the amalgam is liquidized in operation of the lamp, to prevent the amalgam from passing therethrough to the aforementioned lamp envelope arc environment, while permitting mercury vapor to pass therethrough. There is a need, then, that the glass ball retention means, that is, the tubulation pinched portion be such as to facilitate the passage of mercury vapor around the glass balls to and from the amalgam.